This invention relates to a method of forming very small particles of explosive materials. More specifically, it relates to using ultrashort laser pulses to generate energetic nanoparticles. Particles with sizes in the nanometer to micrometer range are highly reactive due to their very large surface-to-volume ratio. There is a need for a method of forming such small particles using a technique that does not produce ignition (initiation of combustion), deflagration (combustion with subsonic flame propagation), or detonation (supersonic combustion with propagating shock wave) when working with highly energetic materials, such as explosives. Standard chemical processes generally do not produce explosives of high purity at particle sizes below a few micrometers.
Avoidance of deflagration or detonation of energetic materials requires exclusion of a source of ignition. Common sources of ignition include thermal sources, electrical sources, and mechanical sources. Many common methods for forming small particles, such as grinding, can lead to ignition, deflagration, and potentially detonation. To avoid deflagration or detonation of energetic materials during the formation of nanoparticles of the material, it is very desirable to employ a method that minimizes exposure to such potential ignition sources. The light-based method of this invention satisfies this need.
Laser machining of explosives has been reported. In general, in machining applications, the production of solid residues such as particles is an undesirable byproduct of the machining process. Therefore, laser machining processes that can produce primarily gas-phase final products, especially chemically inert or nontoxic ones, are especially desirable. Perry et al. report a method for rapid machining with essentially no heat or shock affected zone. In their method, material is removed by a non-thermal mechanism. A combination of multiphoton and collisional ionization creates a critical density plasma in a time scale much shorter than electron kinetic energy is transferred to the lattice. The material is converted from its initial solid state directly into a fully ionized plasma on a time scale too short for thermal equilibrium to be established with the lattice. Hydrodynamic expansion of the plasma eliminates the need for additional techniques to remove material. The material which is removed is rendered inert. The laser pulse converts the explosive material from the solid state to the plasma state; the explosive material is removed from the solid by hydrodynamic expansion of the plasma, wherein the plasma consists of inert gases and no toxic vapor. (M. E. Perry, B. C. Stuart, P. S. Banks, B. R. Myers, and J. A. Sefcik, “Laser Machining of Explosives,” U.S. Pat. No. 6,150,630) Perry et al. reports the use of pulse durations of 5 femtoseconds to 50 picoseconds and fluences that produce a fully ionized plasma for various embodiments of their invention. Peak irradiances greater than 1012 W/cm2 are used. They state that any wavelength laser source can be used provided the beam is focused to achieve a peak irradiance (Watts/cm2) high enough to produce an ionized plasma in vacuum.